Tests to Measure Kidney Function

Laboratory tests for kidney function help to know how well our kidneys are working and detect kidney abnormalities, which mainly include blood urea nitrogen(BUN) test, serum creatinine, creatinine clearance test and testing for proteins: 

Blood urea nitrogen

BUN (blood urea nitrogen) is a blood test which accesses kidney function. Urea is a by-product of protein metabolism and is formed in the liver. Urea is filtered from the blood by the kidneys and excreted into urine. Gastrointestinal tract bleeding, infections or heavy protein diet can cause elevation of BUN. In kidney disorders, blood urea nitrogen increases gradually after kidneys reduce one third to half of their functions.

Normal blood urea nitrogen range in individuals is 3.2-6.1mmol/l.

Serum creatinine test

Serum creatinine is a blood test which is also used to evaluate kidney function. Creatinine is a by-product of creatine which is involved with muscle energy metabolism. Creatinine is filtered from blood by the kidneys and excreted into urine.

Blood creatinine is normally kept quite stable by the kidneys. Creatinine levels in the body become elevated if kidney function reduces by 1/3 to 1/2. Although age and gender affect normal values of creatinine, the stability of the person’s blood creatinine makes it a preferred test to BUN for evaluating kidney problems.

Normal range of serum creatinine is 0.8-1.2 mg/dl in males and 0.6-0.9mg/dl in females.

Creatinine clearance test

Creatinine clearance, also measured as ml/min, compares the level of creatinine in urine with the creatinine level in the blood, usually based on measurements of a 24-hour urine sample and a blood sample drawn at the end of the 24-hour period.

Creatinine is found in stable plasma concentrations. When creatinine is filtered, it is not reabsorbed, and is minimally secreted by the kidneys. The creatinine clearance is used to estimate the glomerular filtration rate, a standard assessment of kidney function.

Creatinine clearance normal range/24 hour urine for adults <40 years old are 90-139 ml/min (males), 80-125 ml/min (females). Over 40 years old normal decreases 6.5 ml/min for each decade of life.

Testing for proteins

Healthy kidneys should filter and absorb proteins instead of excreting them into the urine. A microalbuminuria test detects the presence of protein albumin in the urine—a sign for kidney disease or kidney damage. The preferred test for microalbuminuria is a random spot taken at your doctor’s office or a lab that analyzes both albumin and creatinine levels, called albumin-to-creatinine ratio.

Lab results of 30-299mg protein signal microalbuminuria, one of the first warning sign of diabetic kidney disease. Levels of 300mg or more indicates macroalbuminuria, which is a landmark of early end-stage renal disease (ESRD). However, the diagnosis for both micro and macro albuminuria requires two of three random specimens taken within three to six month timeframe.